Arctic Ocean feels the heat

January 27, 2011By Nicola JonesThis article courtesy of Nature News.

Inflow from the Atlantic is warmest in 2,000 years.

Water flowing into the Arctic Ocean from the Atlantic Ocean is about 2 °C warmer today than it has been for at least 2,000 years, according to a study published in Science1. The findings add to the picture of Earth's warming waters and melting sea ice, and the researchers suggest that the temperature rise is linked to amplification of climate change in the Arctic.

Robert Spielhagen, a palaeoceanographer at the Leibniz Institute of Marine Sciences in Kiel, Germany, and his colleagues focused on the Fram Strait, which runs between Greenland and Norway's Svalbard archipelago, and which hosts the biggest channel of warm water flowing into the Arctic. The current of warm water lies 50 metres below the surface, and can reach a balmy 6 °C in summer — warm in comparison to the frigid Arctic, where icy surface waters can be -2 °C.

To assess the past variations in temperature of this current, Spielhagen and his team took a 46-centimetre-long sample from the sea floor. They scoured the sample for the remains of hard-shelled plankton and compared the number of plankton species known to prefer icy polar waters with the number of those that prefer warmer, subpolar climes. They also measured the ratio of magnesium to calcium in the shells of the plankton, as this varies with water temperature.

Because each half-centimetre slice of the sediment covered a decade or two of history, the researchers could detect only general temperature trends, but both techniques suggested that summer water temperatures averaged around 3.5 °C from AD 1 until the 1850s, at which point they rose about 2 °C. The timing suggests that manmade climate change is the cause of the warming, the authors say.

Past century anomaly

"It's yet another hockey stick," says Joshua Willis, an oceanographer at the Jet Propulsion Laboratory in Pasadena, California, referring to the famous hockey-stick-shaped graph that shows markedly increasing atmospheric temperatures in recent decades. "It's one more piece of evidence that the past 100 years has been an anomaly."

"It's not really surprising, considering that we have seen the recession of the ice cover and a warming of the Arctic over the past 100 years," says Spielhagen. Temperature rises in Atlantic inflow in recent years have come together with increases in the volume of that warm water, he notes. This means that there is probably a large dose of extra heat energy being delivered into the Arctic, he says.

Spielhagen says that extra heat is probably contributing to Arctic ice melt. But oceanographer Ron Kwok, also at the Jet Propulsion Laboratory, says that it is not so simple. There is theoretically enough heat in the Arctic's lower layer of warm, salty water to melt all the sea ice, he says, but this heat has a hard time escaping upwards through the colder, less-salty water lying above it. Arctic ice is certainly thinning and retreating, says Kwok, but it is unclear whether it is warming water or air that is doing this. "How that heat gets delivered to the ice is not well understood."

The study doesn't help to resolve bigger arguments about if and how the planet's main ocean currents are changing as a result of climate change. Some oceanographers have suggested that the 'conveyor belt' that shuttles water around the planet, drives the Gulf Stream and keeps parts of Europe unusually warm, could slow down as Arctic ice melts. Willis recently co-authored a paper arguing that this circulation is so variable from year to year that there simply aren't enough data yet to spot any trends2. Ocean circulation patterns are so complicated, says Willis, that it is possible for the Arctic inflow to get stronger at the same time as the conveyor belt slows down — or vice versa.